Dryer for preparation of dry nanoparticles

ABSTRACT

A system for producing dry nanoparticles from a liquid includes a closed tubing system which incorporates a mister, heater and an electrostatic collector therein. The system is able to produce dry nanoparticles from liquid-suspensions and from solvent solutions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No.15/717,656 filed on Sep. 27, 2017, titled “Dryer For Preparation of DryNanoparticles,” which claims the benefit of priority to similarly titledU.S. Provisional Patent Application Ser. No. 62/408,892 filed on Oct.17, 2016 which is incorporated herein by reference in its entirety.

BACKGROUND Field of Embodiments

The field of invention is spray dryers for creating and/or dryingnanoparticles.

Description of the Related Art

The study of various nanoparticles has been an active area of researchfor their potential applications in numerous fields, such aselectronics, optics, biomedicine and coatings. In order to characterizeand use nanoparticles, they often need to be removed from suspensionwithout affecting the structure and attributes of the nanoparticlestherein. Two known methods for accomplishing this removal include freezedrying and filtering out the solution mechanically. But these methodsare either time consuming and inefficient, or worse, the methods impactthe attributes of the resulting nanoparticles. Freeze drying iseffective and produces little waste, but it is speculated that thisprocess may actually change the structure of the nanoparticles, leadingto changes in their physical properties. Filtering seems to be lessdisruptive to the particles themselves, but it can take an incrediblylong time (considering the fact that it must be continuously observed)and wastes a large percentage of the particles in the solution.Therefore, there is a need in the art for an improved particle dryingprocess that is less disruptive than freeze drying and more efficientthan prior art filtering processes.

The current embodiments described here are directed to a spray dryingapparatus for either creating dry powder from solutions or removingliquid from a dispersion of particles suspended in a fluid. Spray dryersare commonly used to form solid powders from solutions, or fromdispersions of solid particles in a liquid. In the latter case, spraydryers may have advantages over conventional drying in terms of reducingor eliminating particle aggregation. The preparation of nanoparticlesfrom solutions is a critical step in numerous processes across manyindustries including, but not limited to processes for forming:pharmaceuticals, nutraceuticals, pesticides, polymer colloids, molecularcrystals, and other materials.

Two exemplary prior art spray dryers are shown in FIGS. 1a and 1b andfurther described in the Technical data sheets for Buchi products “NanoSpray Dryer B-90” and “Mini Spray Dryer B-290” the contents of which areincorporated herein by reference. With respect to FIG. 1a , inlet air 6(drying gas) is delivered to a heater 1 while droplets are introducedthrough an ultrasonic spray head at 2. At 3, there is a conductive heatexchange between drying gas and droplets and resulting particles arecollected at electrostatic particle collector 4. An outlet filter 5collects fine particles not collected at 4 to protect the user andenvironment. With respect to FIG. 1b , the process steps are generallythe same as those described in FIG. 1a , except the particle collectionat 4 is achieved using cyclone technology which is well known (see,e.g., U.S. Pat. No. 2,911,036 to Lazar et al.)

Prior art spray dryers are not entirely self-contained, thus resultingin the loss of material through venting, safety risks due to release ofnanoparticles to the outside, and contamination of sensitive materialfrom outside contaminants. Further, the particle yield suffers due tothe fact that there is only a single pass through the collector and as aresult of ventilation. There is a need in the art for a self-containedspray dryer to address these issues in the prior art and improve theefficiency, safety and yield of nanoparticles.

SUMMARY OF THE EMBODIMENTS

In a first exemplary embodiment herein, a system for dryingliquid-suspended nanoparticles includes: a closed tubing systemincluding an access port for receiving a liquid solution therein,wherein the liquid solution contains suspended nanoparticles therein; amister for transforming the liquid solution into an aerosol containingdroplets, wherein the droplets contain liquid and suspendednanoparticles; a heater for heating a first portion of the closed tubingsystem, creating an updraft therein, and causing the aerosol to movethrough at least a second portion of the closed tubing system andfurther causing the evaporation of at least some liquid from thedroplets in the aerosol, thereby leaving dried nanoparticles; and anelectrostatic collector comprised of one or more electrodes located in athird portion of the closed tubing system, wherein the driednanoparticles are collected at the one or more electrodes of theelectrostatic collector as the aerosol flows therethrough.

In a second exemplary embodiment, a system for creating drynanoparticles from a solution includes: a closed tubing system includingan access port for receiving the solution therein, wherein the solutioncontains materials therein for forming nanoparticles; a mister fortransforming the solution into an aerosol containing droplets, whereinthe droplets contain solvent and the materials therein; a heater forheating a first portion of the closed tubing system, creating an updrafttherein, and causing the aerosol to move through at least a secondportion of the closed tubing system and further causing the evaporationof at least some liquid from the droplets in the aerosol and formationof dried nanoparticles from the materials; and an electrostaticcollector comprised of one or more electrodes located in a third portionof the closed tubing system, wherein the dried nanoparticles arecollected at the one or more electrodes of the electrostatic collectoras the aerosol flows therethrough.

In a third exemplary embodiment, a process for drying liquid-suspendednanoparticles includes: receiving a liquid solution into a closed tubingsystem, wherein the liquid solution contains suspended nanoparticlestherein; misting the liquid solution into an aerosol containingdroplets, wherein the droplets contain liquid and suspendednanoparticles; heating a first portion of the closed tubing system,creating an updraft therein, and causing the aerosol to move through atleast a second portion of the closed tubing system and further causingthe evaporation of at least some liquid from the droplets in theaerosol, thereby leaving dried nanoparticles; and collecting, by anelectrostatic collector comprised of one or more electrodes located in athird portion of the closed tubing system, the dried nanoparticles atthe one or more electrodes of the electrostatic collector as the aerosolflows therethrough.

BRIEF SUMMARY OF FIGURES

FIGS. 1a and 1b are schematics of prior art systems;

FIGS. 2a and 2b are schematic diagram of nanospray dryer in accordancewith an embodiment of the present invention;

FIG. 3 is a schematic diagram of nanospray dryer in accordance with anembodiment of the present invention;

FIG. 4 is a schematic diagram of nanospray dryer depicting applicationof a potential in accordance with a first configuration; and

FIG. 5 is a schematic diagram of nanospray dryer depicting applicationof a potential in accordance with a second configuration.

DETAILED DESCRIPTION

In a first preferred embodiment, a liquid solution or suspension(hereafter “liquid”) 15 is introduced to a self-contained nanospraydryer 10 of FIG. 2a , either through an automated syringe pump ormanually. Here it is first converted to an aerosol or mist by anebulizer or atomizer (hereafter “mister”) 20. For example, the liquidcould be an aqueous suspension of polystyrene nanoparticles, or theliquid could include an organic dye perylene dissolved in an organicsolvent such as liquid ethanol. One skilled in the art recognizes thatthe liquid may be selected from numerous aqueous and organic solventsincluding, but not limited to ethanol, methanol and acetone.Additionally, the nanospray dryers 10 described herein createnanoparticles in two ways. In the first, the particles are alreadyformed, i.e., are suspended in the liquid or solvent. These particlesare encapsulated inside the aerosol droplets and collected after drying.In the second method for nanoparticle creation, the aerosol consists ofa solution of the desired material, i.e., the material is completelydissolved in the solvent, and the material in each aerosol droplet formsthe nanoparticle as it dries. Nanoparticles as referenced hereingenerally refers to a size of the resulting dried and collectedparticles of less than 1000 nm and may include: nanocrystals,nanofibers, nanotubes, nanobeads, and nanoclusters.

The mister 20 may be an ultrasonic wave nebulizer, which uses a highfrequency ultrasonic wave to form aerosol droplets from the liquid.Alternatively, a nebulizer based on vibrating mesh technology could alsobe used. The resulting aerosol 25 consists of liquid droplets with sizeson the order of approximately 0.1 to 100 microns. These droplets cancontain suspended solid particles or a solution which will later formthe solid particles. The mister 20 may also be one of the variousapparatuses available for generating aerosols or mists, includingatomizer nozzles and electrospray generators.

These droplets are carried to the recirculating tube 30. The tube can bemade of metal, plastic or glass, and the inner surface can be treated orcoated, for example, with Teflon™, to minimize adsorption of aerosoldroplets or nanoparticles. The tube 30 is completely sealed in a closedloop. The droplets are carried through the left vertical section of thetube 30 a by convection updraft caused by a heating unit 35. The heatingunit 35 may be an infrared lamp for heating a portion of the metaltubing or a portion of metal in contact with a hotplate, or anelectrical heating unit inside the tube. Heater selection is not socritical as the requirement that the internal temperature of therecirculating tube not exceed 50 degrees Celsius to avoid degradation ofthe nanoparticles. In most cases, there is complete drying of theaerosol particles, due to the heat in the tube, over distances of lessthan 1 meter. Accordingly, depending on the dimensions of therecirculating tube 30, the water or solvent in which the particles orsolute are suspended or dissolved will likely have evaporated due to theheater, shortly after entering the top horizontal section 30 b and priorto reaching the collector 45, leaving only nanoparticles ranging in sizefrom 1 to 1000 nanometers. By way of example only, 10 cm diameter tubinghas been implemented on one or more of the exemplary embodiments.

Next, the nanoparticles are collected by an electrostatic collector 45located within section 30 b. It has been determined that while thenanoparticles are naturally charged during the original aerosolformation, depending on the structure of the electrostatic collector 45,large electric fields between electrodes of the electrostatic collector45 induce electric dipoles in the nanoparticles, which are thenattracted to one of the electrodes due to the inhomogeneous field.Fields on the order of 2.0×10⁵ V/m to 2.5×10⁵ V/m may be most effectivein collecting particles without causing a corona discharge at theelectrodes, which could possibly damage the nanoparticles. An exemplaryelectrostatic collector 45 useful in the first preferred embodiment is aset of interdigitated electrodes formed using a series of oppositelycharge plates. Using the recirculating process described herein, whereinthe nanoparticles may make multiple passes through the tube 30, it hasbeen shown that in a single pass through the device, a collection rateof 40% could be achieved with the interdigitated electrodes. For 10passes, we estimate that a 99% collection rate could be achieved withthe interdigitated electrodes, provided that any possible inefficienciesare sufficiently reduced, such as adsorption of particles on the tubewalls 30 or moisture trap 60.

Alternatively, one skilled in the art recognizes that otherelectrostatic collector configurations may also be contemplated,including dual plate electrodes located within the tube, multipleparallel collector plates, as well as collectors based on the generationof a radial electric field using rod/cylindrical collector platesdesigns. A key driving factor with respect to the configuration of theelectrostatic collector is the requirement for generation of a largeenough electric field to induce electric dipoles in the nanoparticles,while avoiding corona discharge, to facilitate attraction to thecollector electrodes.

FIG. 2b illustrates a slightly different configuration, wherein thedroplets pass through a charging grid 40, prior to reaching theelectrostatic collector 45, which is held at a high enough electricpotential to induce a corona discharge and charge the aerosol droplets.As in FIG. 2a , the charged nanoparticles are then collected on anelectrostatic collector 45 wherein a potential applied thereto is ofopposite polarity as that applied to the charging grid 40. Theelectrostatic interaction causes the particles to collect on the chargedplates 45.

Further to the configurations in both FIGS. 2a and 2b , the air flow 50continues down the tube on the right side 30 c, where it is cooled inorder to allow any remaining water/solvent vapor to condense and becollected at the moisture trap 60 along the bottom tube section 30 d.This cooling can be natural or aided by either an external or internalcooling element 55 located on the right side of the tube, i.e., oppositethe heating side of the tube 30. In order to enhance the evaporationrate, the initial liquid may be heated, and a desiccant can be includedin the recirculating path in order to remove moisture. Depending on thenumber of passes the air flow 50 has made through the tube 30, the airflow 50 contains varying amounts of remaining liquid/solvent and/ornanoparticles. Nanoparticles that have not been trapped by thecollection plates 45 will continue to circulate through the tube 30 andbe trapped on subsequent passes, thus increasing yield.

Once the spray drying is complete, in a first exemplary embodiment, thecollector plates 45 can be physically removed from the system through anaccess hatch or equivalent thereof in order to retrieve the nanoparticlesample. A reverse potential and/or mechanical vibrations may be appliedto the collection plate in order to help dislodge the particles.Alternatively, it is further contemplated that a reverse potentialand/or mechanical vibration may be applied to the collector plates 45without requiring removal from the tubing 30. In this alternativearrangement, a nanoparticle trap 65 may be incorporated into the system10.

Accordingly, the enclosed dryer 10 mitigates the danger of nanoparticlesbeing released into the atmosphere and reduces contamination of thesample from outside contaminants. Further, the recirculation of air flowcontaining particles will increase the yield of nanoparticles obtained.

Although the shape of tubing 30 is shown in FIGS. 2a and 2b to berectangular, the embodiments are not so limited. Referring to FIG. 3, analternative configuration illustrates a dryer 100 having a circular tube130. Additionally, FIG. 3 also illustrates that the nebulizer (or otherdroplet forming apparatus) 120 may access the tube 130 from outside thecircumference of the tube 130 (as compared to the inside accessillustrated in FIGS. 2a and 2b ).

In alternative embodiments, additional electrostatic control ofnanoparticles within the tube 30 may be achieved, as needed, through theuse of one or both of:

-   -   1. Application of an electric potential 70 to one or more        sections of tube 30 that is of the same polarity to the charge        on the particles in order to keep the particles from        accumulating on the walls of tube 30, as shown in FIG. 4.    -   2. Application of pulsed electric fields at periodic intervals        around the path of the particles through tube 30 to control        their motion toward the electrostatic collector plates 45, as        shown in FIG. 5. The potentials can be applied directly to the        tube or applied to metal bands 80 surrounding the tube at        intervals.        These electric potentials, as well as those applied to the        charging grid and collector plates, are supplied by (high)        voltage power supplies external to the drying unit and connected        via insulated copper wire.

One or more of the embodiments described herein has formed nanoparticlesfrom solutions having initial concentrations of up to 14.6 millimolar(mM), producing particles with sizes on the order of 500 nm in radius.One skilled in the art will appreciate that particle size may becontrolled through variance of the initial solution concentration. Forparticles that were already in solution, concentrations up to 11.3 mg/mlhave been successfully used for spray drying, but one skilled in the artwill appreciate that higher concentrations are achievable with one ormore of the system embodiments described herein.

One skilled in the art will understand the various specific components,e.g., product types and substitutes therefor, which may be used toachieve heating, cooling, charging and the like. Additionally, oneskilled in the art will recognize various types of tubing that may beused. Further, scaling of the system is within the knowledge of thoseskilled in the art. These variations are considered to be within thescope of the invention.

The invention claimed is:
 1. A process for drying liquid-suspendednanoparticles comprising: receiving a liquid solution into a closedtubing system, wherein the liquid solution contains suspendednanoparticles therein; misting the liquid solution into an aerosolcontaining droplets, wherein the droplets contain liquid and suspendednanoparticles; heating a first portion of the closed tubing system,creating an updraft therein, and causing the aerosol to move through atleast a second portion of the closed tubing system and further causingthe evaporation of at least some liquid from the droplets in theaerosol, thereby leaving dried nanoparticles; and collecting, by anelectrostatic collector comprised of one or more electrodes located in athird portion of the closed tubing system, the dried nanoparticles atthe one or more electrodes of the electrostatic collector as the aerosolflows therethrough.
 2. The process according to claim 1, wherein theheating a first portion of the closed tubing system further comprisesheating to a temperature within the closed tubing system equal to orless than 50 degrees Celsius.
 3. The process according to claim 1,further comprising recirculating any remaining aerosol through theclosed tubing at least one additional time and collecting any additionaldried nanoparticles therefrom at the electrostatic collector.
 4. Theprocess according to claim 1, further comprising receiving any remainingaerosol in a fourth portion of the closed tubing for cooling thereinwherein any additional moisture generated from cooling is caught in amoisture trap at the end of the fourth portion of the closed tubing. 5.The process according to claim 3, further comprising receiving anyremaining aerosol in a fourth portion of the closed tubing for coolingtherein after the at least one additional recirculation, wherein anyadditional moisture generated from cooling is caught in a moisture trapat the end of the fourth portion of the closed tubing.
 6. A process fordrying liquid-suspended nanoparticles comprising: receiving a liquidsolution into a closed tubing system, wherein the liquid solutioncontains suspended nanoparticles therein; misting the liquid solutioninto an aerosol containing droplets, wherein the droplets contain liquidand suspended nanoparticles; heating a first portion of the closedtubing system, creating an updraft therein, and causing the aerosol tomove through at least a second portion of the closed tubing system andfurther causing the evaporation of at least some liquid from thedroplets in the aerosol, thereby leaving dried nanoparticles; passingthe aerosol through a charging grid for charging the aerosol droplets;and collecting, by an electrostatic collector comprised of one or moreelectrodes located in a third portion of the closed tubing system, thedried nanoparticles at the one or more electrodes of the electrostaticcollector as the aerosol flows therethrough.
 7. The process according toclaim 6, wherein the heating a first portion of the closed tubing systemfurther comprises heating to a temperature within the closed tubingsystem equal to or less than 50 degrees Celsius.
 8. The processaccording to claim 6, further comprising recirculating any remainingaerosol through the closed tubing at least one additional time andcollecting any additional dried nanoparticles therefrom at theelectrostatic collector.
 9. The process according to claim 6, furthercomprising receiving any remaining aerosol in a fourth portion of theclosed tubing for cooling therein wherein any additional moisturegenerated from cooling is caught in a moisture trap at the end of thefourth portion of the closed tubing.
 10. The process according to claim8, further comprising receiving any remaining aerosol in a fourthportion of the closed tubing for cooling therein after the at least oneadditional recirculation, wherein any additional moisture generated fromcooling is caught in a moisture trap at the end of the fourth portion ofthe closed tubing.
 11. The process according to claim 6, comprisingapplying a first potential to the charging grid and applying a secondpotential to the electrostatic grid, where in the first and secondpotential are opposite in polarity.
 12. A process for dryingliquid-suspended nanoparticles and collecting dried nanoparticlescomprising: receiving a liquid solution into a closed tubing system,wherein the liquid solution contains suspended nanoparticles therein;misting the liquid solution into an aerosol containing droplets, whereinthe droplets contain liquid and suspended nanoparticles; heating a firstportion of the closed tubing system, creating an updraft therein, andcausing the aerosol to move through at least a second portion of theclosed tubing system and further causing the evaporation of at leastsome liquid from the droplets in the aerosol, thereby leaving driednanoparticles; collecting, by an electrostatic collector comprised ofone or more charging plates located in a third portion of the closedtubing system, the dried nanoparticles at the one or more chargingplates of the electrostatic collector as the aerosol flows therethrough;removing the charging plates from the closed tubing system to collectthe dried nanoparticles therefrom.
 13. The process according to claim12, wherein the heating a first portion of the closed tubing systemfurther comprises heating to a temperature within the closed tubingsystem equal to or less than 50 degrees Celsius.
 14. The processaccording to claim 12, further comprising recirculating any remainingaerosol through the closed tubing at least one additional time andcollecting any additional dried nanoparticles therefrom at theelectrostatic collector.
 15. The process according to claim 12, furthercomprising receiving any remaining aerosol in a fourth portion of theclosed tubing for cooling therein wherein any additional moisturegenerated from cooling is caught in a moisture trap at the end of thefourth portion of the closed tubing.
 16. The process according to claim14, further comprising receiving any remaining aerosol in a fourthportion of the closed tubing for cooling therein after the at least oneadditional recirculation, wherein any additional moisture generated fromcooling is caught in a moisture trap at the end of the fourth portion ofthe closed tubing.
 17. The process according to claim 12, furthercomprising passing the aerosol through a charging grid for charging theaerosol droplets prior to reaching the electrostatic collector.
 18. Theprocess according to claim 17, comprising applying a first potential tothe charging grid and applying a second potential to the electrostaticgrid, where in the first and second potential are opposite in polarity.19. The process according to claim 12, further comprising applying areverse potential to the removed charging plates to dislodge the driednanoparticles therefrom.